(a) Field of the Invention
The present invention relates to a superconducting cable.
Particularly, the invention relates to a superconducting cable that does not have a vacuum thermal insulation structure, or a superconducting cable that has a vacuum thermal insulation structure and that can maintain the thermal insulation performance even if the vacuum condition is destroyed.
(b) Description of Related Art
The superconducting cable as shown in FIG. 8 is a proposed conventional superconducting cable. FIG. 8 is a sectional view of a three core-in-one type superconducting cable having a structure in which three cores 110 are housed in a thermal insulation pipe 600.
A cable core 110 is equipped with a former 111, a superconductor layer 112, an electrical insulation layer 113, a shielding layer 114, and a protective layer 115, in an enumerated order from the center thereof. The conductor layer 112 is formed by spirally winding superconducting wires in multiple layers on the former 111. Generally, a superconducting wire has a structure of tape-like shape in which a plurality of filaments consisting of an oxide superconducting material are arranged in a matrix such as a silver sheath. The insulation layer 113 is formed by winding an insulation paper such as a semisynthetic insulation paper. The shielding layer 114 is formed by spirally winding a superconducting wire, which is similar to the conductor layer 112, on the electrical insulation layer 113. An insulation paper or the like is used as the protective layer 115.
On the other hand, a thermal insulation pipe 600 is structured such that a thermal insulation material (not illustrated) is arranged between the double pipes consisting of an inner pipe 610 and an outer pipe 620, the inside of the double pipe is evacuated. An anticorrosion layer 630 is formed outside the thermal insulation pipe 600. Furthermore, the space existing inside the former 111 (in the case where the former is hollow) and a space between the inner pipe 610 and the cores 110 are filled with a coolant such as liquid-nitrogen or the like which circulates therein so that the thermal insulation pipe may be in a usable condition in a state of the insulation layer 113 being impregnated with the coolant.    [Patent document 1] Japanese Patent Application Publication No. 2002-140944 (FIG. 2)
However, there have been the following problems with respect to the above-mentioned superconducting cable.
(1) The vacuum thermal insulation structure needed for the superconducting cable results in a large-sized cable. In order to make a vacuum thermal insulation structure, it is necessary to use a thermal insulation pipe having a double pipe structure and to evacuate the space between the inner and the outer pipes of the thermal insulation pipe. Therefore, the thickness of the thermal insulation pipe increases, and particularly the outer diameter of the superconducting cable becomes large-sized. Accordingly, the manufacturing cost of the superconducting cable becomes high.
(2) The maintenance and control of the vacuum performance of the thermal insulation pipe are complex. Using an evacuated thermal insulation pipe having a double pipe structure requires the maintenance and control of the vacuum performance in the manufacture, construction and operation steps of the superconducting cable. Particularly, when a malfunction occurs to the vacuum performance of the thermal insulation pipe, it takes a large amount of time to recover the pre-determined vacuum condition once again. Depending on the conditions, it may be difficult to recover the pre-determined vacuum condition within a given period of time, which might result in failure of maintaining the coolant temperature, thereby causing a lack of a power transmission property.